Anharmonic coupling between fundamental modes in tetramethylurea

Abstract

In situ high pressure Raman spectra of tetramethylurea have been measured up to 25 GPa, liquid-solid and solid-solid phase transitions were detected at 0.2 GPa and 7.4 GPa, respectively. An unprecedented spectral phenomenon is the observation of a Fermi resonance between the fundamental modes. An exponential relationship between the intensity and the frequency difference was concluded. Pressure provides us a new way to study the correlation between Fermi resonance parameters.

Pressure‐induced Fermi resonance between fundamental modes in 7,7,8,8‐tetracyanoquinodimethane

“…P is the external pressure, and I f and I a represent the intensity of C―H rocking and C―H scissoring vibration, respectively. The pressure coefficient value of 1.38 is consistent with our previous prediction of anharmonic coupling between fundamental modes . This is another spectral evidence of Fermi resonance between fundamental modes.…”

“…(b)). In the previous report, we concluded a mathematic relationship of intensity ratio and pressure between fundamental Fermi doublets. We fitted the experimental curves in the decoupling process and got the parameter values using Eqn .…”

“…Attempts have been made to study the relationships between coupling parameters and external pressure. Furthermore, a pressure‐induced anharmonic coupling between the fundamental mode in tetramethylurea (TMU) was reported for the first time . The mathematical relationship between the frequency difference and intensity with pressure was established.…”

High Pressure Spectroscopic Investigation on Proton Transfer in Squaric Acid and 4,4′-Bipyridine Co-crystal

“…We tend to assign the triple bands to a vibrational anharmonic coupling (Fermi resonance) between fundamental C = C stretching mode and a combination band of 658 and 998 cm −1 . Fermi resonance, a special case of mechanical coupling, is very sensitive to the detailed structure and local environment of the molecule 56, 57 . Due to the intermolecular hydrogen bonds and proton transfer, the molecular structure of BP in co-crystal will generally deviate from its crystal structure.…”

Pressure-induced Fermi resonance between fundamental modes in phthalic anhydride

“…In addition, a double Fermi resonance has been proposed in hexachloroethane [18]. Furthermore, a pressure-induced Fermi resonance between fundamental modes were reported in tetramethylurea and 7,7,8,8-tetracyanoquinodimethane [19,20], where an exponential relationship between Raman intensity ratio to pressure was proposed. In the latest study, a Fermi resonance between the lowest frequency Ag symmetry peaks (ν4 and ν5) between ∼10 and ∼30 GPa has been observed in ilmenite [21].…”